Steven Dietz has been ACT Theatre's patron playwright for the past several years, and a highly prolific one. The theater has produced 10 of his plays in the past two decades, many of them marital dramas about trust and deceit (Fiction, Trust, Becky's New Car), two of Dietz's regular themes.

But good things happen when Dietz turns his eyes beyond the four walls of domesticity and writes about individuals paddling through large tides of politics and history. His best plays, such as God's Country and Lonely Planet, have set their characters in the context of white-supremacy movements and the beginning of the AIDS epidemic, respectively. Now with Yankee Tavern, he's facing up to 9/11, conspiracies, and spooks.

He's set himself an interesting challenge: to write a spy thriller for the stage, set entirely in the confines of a broken-down bar, meaning all the action has to happen in the dialogue. (Or most of it anyway; every broken-down bar has its gun hiding behind the counter.) Yankee Tavern largely succeeds, partly because of the script and largely because of the performances—particularly that of Charles Leggett, who commands the first act as Ray, a conspiracy-spouting curmudgeon with a stentorian voice and infinite confidence who splits his life between the barroom and the ratty, mostly abandoned apartments upstairs. Ray storms up and down and around the bar, haranguing call-in radio shows and the bar's baby-faced and mildly exasperated owner, Adam (Shawn Telford), whose father founded the place. Now Adam is finishing his thesis, getting married, and ready to leave the Yankee Tavern (and his old, paranoid family friend) behind.

But stuff happens, as it must, including the arrival of a steely stranger who always orders two Rolling Rocks but drinks only one (R. Hamilton Wright) and Adam's mysterious professor, who might be tangled up in the same shadowy government conspiracies that both terrify and fascinate Ray.

The play tromps along enjoyably before revealing its secret teeth. Ray, like all red-blooded American conspiracy theorists, has a special fondness for the oddities surrounding the 9/11 attacks. And all the details he recites are part of the real-life public record: that the towers were designed to withstand airplane impacts; that jet fuel does not burn hot enough to melt steel; that the towers should not have collapsed so quickly, completely, or neatly into their footprints; that a passport, allegedly belonging to one of the hijackers, was discovered in immaculate condition in a pile of rubble, having passed unharmed through the explosion and collapsed building; and so on.

Dietz doesn't let Ray go so far as to claim that 9/11 was an inside job. But he drills down on things about the attacks that are difficult to explain or ignore. Yankee Tavern holds its mysteries close in a pleasurable and unsettling embrace.